I have a situation where there is a corrupt WAV file from which I'm trying to recover data.
My colleagues have sliced up the large WAV file into smaller WAV files with proper headers. This has produced some interesting results.
Sliced into 1MB segments we get these results:
The first wave file segment is all noise.
The second wave file segment is distorted.
The third wave file segment is clear.
This pattern is repeated for the entire length of the file (after it's been broken into smaller files).
For 20MB slices:
The first wave file segment is all noise.
The second wave file segment is clear.
The third wave file segment is distorted.
Again, this pattern is repeated for the entire length of the file (after it's been broken into smaller files).
Would anyone know why this is occurring?
Assuming the WAV contains uncompressed (raw) samples, recovery should be easy. You need to know the sample format. For example: 16 bits, two channels, 44100 Hz (which is cd quality). Because one of the segments is okay, then you can look at this to figure out what the right values are.
Then just open the WAV using these values in, e.g., Adobe Audition (formerly Cool Edit), or any other wave editor that supports import of raw data.
Edit: Okay, now to answer your question. Some segments are clear, because then the alignment is right. Take the cd quality again, as I described before. The bytes of one sample look like this:
left_channel_high | left_channel_low | right_channel_high | right_channel_low
(I'm not sure about the ordering here! But it's just an example.) So the first data byte had better be the most significant byte of the left channel, or else you'll end up with fragments of two samples being interpreted as one whole sample:
left_channel_low | right_channel_high | right_channel_low || left_channel_high
-------------------part of first sample------------------ || --second sample--
You can see that everything "shifted" here, which happens because the size of your file slices is not a multiple of the sample size in bytes.
If you're lucky, this just causes the channels to be swapped. If you're unlucky, high and low bytes get swapped. Interestingly, this does lead to kind-of recognizable, but severely distorted audio.
What puzzles me is that the pattern you report repeats in blocks of three. From the above, I'd expect either two or four. Perhaps you are using an unusual sample format, such as 24-bits (3 bytes)?
Related
I am working on a project in which I have to merge two 8bits .wav files using C and i still have no clue how to do it.
I have read about wav files and I want to start by reading one of the files.
There's one thing i didn't understand:
Let's say i have an 8bit WAV audio file, And i was able to read (even tho I am still trying to) the Data that starts after the 44 byte, I will get numbers between 0 and 255 logically.
My question is:
What do those numbers mean?
If I get 255 or 0 what do they mean?
Are they samples from the wave?
Can anyone please explain?
Thanks in advance
Assuming we're not dealing with file format issues, getting values between 0 and 255 means that the audio samples are of unsigned eight-bit format, as you have put it.
One way of merging data would consist of reading data from files into buffers, arrays a and b and summing them value by value: c[i] = a[i] + b[i]. By doing so, you'd have to take care of the following:
length of the files may not be equal
on summing the unsigned 8-bit buffers, such as yours will almost certainly overflow
This is usually achieved using a for loop. You first get the sizes of the chunks. Your for loop has to be written in such a way that it neither reads past the array boundary, nor ignores what can be read. For preventing overflows you can either:
divide values by two on reading
or
read (convert) into a format which wouldn't overflow, then normalize and convert the merged data back into the original format or whichever format desired.
For all particulars of reading from and writing to a .wav format file you may use some of the existing audio file libraries, or write your own routine. Dealing with audio file format is not a trivial thing, though. Here's a reference on .wav format.
Here are few audio file APIs worth of looking at:
libsndfile
sndlib
Hope this can help.
See any good guide to WAVE for information on the format of samples in the data chunk, such as this one I found: http://www.neurophys.wisc.edu/auditory/riff-format.txt
Relevant excerpts:
In a single-channel WAVE file, samples are stored consecutively. For
stereo WAVE files, channel 0 represents the left channel, and channel
1 represents the right channel. The speaker position mapping for more
than two channels is currently undefined. In multiple-channel WAVE
files, samples are interleaved.
Data Format of the Samples
Each sample is contained in an integer i. The size of i is the
smallest number of bytes required to contain the specified sample
size. The least significant byte is stored first. The bits that
represent the sample amplitude are stored in the most significant bits
of i, and the remaining bits are set to zero.
For example, if the sample size (recorded in nBitsPerSample) is 12
bits, then each sample is stored in a two-byte integer. The least
significant four bits of the first (least significant) byte is set to
zero.
The data format and maximum and minimums values for PCM waveform
samples of various sizes are as follows:
Sample Size Data Format Maximum Value Minimum Value
One to Unsigned 255 (0xFF) 0
eight bits integer
Nine or Signed Largest Most negative more bits
integer i positive value of i
value of i
N.B.: Even if the file has >8 bits of audio resolution, you should read the file as an array of unsigned char and reconstitute the larger samples manually as per the above spec. Don't try to do anything like reading the samples directly over an array of native C ints, as their layout and size is platform-dependent and therefore should not be relied upon in any code.
Note also that the header is not guaranteed to be 44 bytes long: How can I detect whether a WAV file has a 44 or 46-byte header? You need to read the length and process the header based on that, not any assumption.
I've read through numerous articles on GIF LZW decompression, but I'm still confused as to how it works or how to solve, in terms of coding, the more fiddly bits of coding.
As I understand it, when I get to the byte stream in the GIF for the LZW compressed data, the stream tells me:
Minimum code size, AKA number of bits the first byte starts off with.
Now, as I understand it, I have to either add one to this for the clear code, or add two for clear code and EOI code. But I'm confused as to which of these it is?
So say I have 3 colour codes (01, 10, 11), with EOI code assumed (as 00) will the byte that follows the minimum code size (of 2) be 2 bits, or will it be 3 bits factoring in the clear code? Or is the clear code/EOI code both already factored into the minimum size?
The second question is, what is the easiest way to read in dynamically sized bits from a file? Because reading an odd numbers of bits (3 bits, 12 bits etc) from an even numbered byte (8) sounds like it could be messy and buggy?
To start with your second question: yes you have to read the dynamically sized bits from an 8bit bytestream. You have to keep track of the size you are reading, and the number of unused bits left from previous read operations (used for correctly putting the 'next byte' from the file).
IIRC there is a minimum code size of 8 bits, which would give you a clear code of 256 (base 10) and an End Of Input of 257. The first stored code is then 258.
I am not sure why you did not looked up the source of one of the public domain graphics libraries. I know I did not because back in 1989 (!) there were no libraries to use and no internet with complete descriptions. I had to implement a decoder from an example executable (for MS-DOS from Compuserve) that could display images and a few GIF files, so I know that can be done (but it is not the most efficient way of spending your time).
For the purposes of this example, suppose there exist 2 binary files A and B, each containing a variation of, say, youtube video, where
A contains a 5 second ad
B contains no ad
With the exception for the ad, A contains the same content as B
Total length of file A is 60 seconds
Total length of file B is 55 seconds
As a general rule, if we were to compare bits patterns of each file, would we arrive to the same conclusion: files contain 55 seconds worth of common bits?
If we extend the problem further, say to the world of 2 jars, the only difference between which are comments, would it be appropriate to compare the order of bits and based on what we find, determine the degree of likeness?
It's easy to determine whether files are identical or not. Will the approach of comparing bits help accurately determine the degree to which files are close to one another?
The question is not about video files, but rather a general binary files. I mention video file above for example purposes only.
It depends on the file-format, but in your examples — no, probably not.
Video with and without initial ad: videos are usually encoded by breaking them into small time-blocks, and then encoding and compressing those blocks; if you insert an ad at the beginning, then you will most likely cause the block-transitions to happen at different time offsets within the main video.
Jar-file with and without comments (or with different comments): same story; changing the length of a comment within a file will affect the splitting of the entire file into compressible blocks, so all blocks after an altered comment will be compressed differently. (This is, of course, assuming that the jar-file actually includes the comments. Just because comments were in the source-code, that doesn't mean the jar-file will have them; that depends on compiler settings and so on.)
Most video compression these days is done with lossy algorithms. The compression is done both within a frame and BETWEEN frames. If the extra video frames added in your "A" video "leak" into the original movie because of the inter-frame compression, then by definition your two video files will be different videos, even though logically they're the same movie with 5 seconds of ad tacked onto the front. The compression algorithm will have merged 1 or more frames of the two videos into a hybrid of the two, and this fundamentally changes things.
I have an interesting question today.
I need to convert some pokemon audio files to a list of 8-bit samples (0-255 values). I am writing an assembly routine on the MC6800 chipset that will require these sounds to be played. I plan on including an array with the 8-bit samples that the program will loop through when a function is called.
Does anyone know a way to convert audio files (wav/mp3) into a list of comma separated 8-bit text sample values? Or anything of this relative method?
Thank you so much in advance!
You can use the command-line "sox" tool or the Audacity audio editor to convert the file to a raw, unsigned 8-bit mono audio file.
In Audacity 1.3 or higher, open the audio then select Export, choose "Wave, AIFF, and other uncompressed types" as the format, then click Options... - then choose "Other..." for the Format, "RAW" for the Header, and Signed 8-bit PCM as the encoding. (Sorry, unsigned isn't available.)
From the command line, try sox with -c 1 for 1 channel, -t raw for no header, -u for unsigned linear, and -1 for 1 byte per sample.
Then you can use a tool like "hexdump" to dump out the bytes of the file as numbers and paste them into your code.
If sox doesn't have it, you will have to use it to generate raw (headerless) files and convert the raw files to comma-separated yourself.
EDIT: sox has "Raw textual data" as one of its formats, from the web page. You can make it convert your sound files to unsigned 8-bit linear samples in a first pass and then probably get exactly the output you want using this option for output.
For .wav it is a very simple process. You can find the .wav specification easily with a google search. It comprises a header then simply raw samples. You should read the header first, then loop through all the samples. Usually they are 16 bit samples, so you want to normalize them from the range -32768 to 32767 to your 0-255 range. I suggest simple scaling at first. If that's not successful maybe find the actual min and max amongst the samples and adjust your scale accordingly.
Well a lot depends on your audio format. The wave format, for example, consists of uncompressed interleaved PCM data.
ie for an 8-bit stereo file each sample will be arranged as follows.
[Left Sample 1][Right Sample 1][Left Sample 2][Right Sample2]...[Left Sample n][Right sample n].
ie each 8 bit stereo sample is stored in 2 bytes. 1 for the left channel and 1 for the right. This is the data format your sound hardware will most likely require.
A 16 or 24-bit audio file will work in each way but the left and right samples will be 2 or 3 bytes each, respectively.
Obviously a wave file has a load of extyra information in it. It follows the RIFF format. You can find info on it and the "chunks" wave files use at places such as www.wotsit.org.
To decompress an MP3 is more complicated. You are best off getting hold of a decompressor and running it on the MP3 encoded audio. IT will spit out PCM data as above from the other side.
I'm trying to read a 16-bit greyscale TIFF file (BitsPerSample=16) using a small C program to convert into an array of floating point numbers for further analysis. The pixel data are, according to the header information, in a single strip of 2048x2048 pixels. Encoding is little-endian.
With that header information, I was expecting to be able to read a single block of 2048x2048x2 bytes and interpret it as 2048x2048 2-byte integers. What I in fact get is a picture split into four quadrants of 1024x1024 pixels each, the lower two of which contain only zeros. Each of the top two quadrants look like I expected the whole picture to look: alt text http://users.aber.ac.uk/ruw/unlinked/15_inRT_0p457.png
If I read the same file into Gimp or Imagemagick, both tell me that they have to reduce to 8-bit (which doesn't help me - I need the full range), but the pixels turn up in the right places: alt text http://users.aber.ac.uk/ruw/unlinked/15_inRT_0p457_gimp.png
This would suggest that my idea about how the data are arranged within the one strip is wrong. On the other hand, the file must be correctly formatted in terms of the header information as otherwise Gimp wouldn't get it right. Where am I going wrong?
Output from tiffdump:
15_inRT_0p457.tiff:
Magic: 0x4949 Version: 0x2a
Directory 0: offset 8 (0x8) next 0 (0)
ImageWidth (256) LONG (4) 1<2048>
ImageLength (257) LONG (4) 1<2048>
BitsPerSample (258) SHORT (3) 1<16>
Compression (259) SHORT (3) 1<1>
Photometric (262) SHORT (3) 1<1>
StripOffsets (273) LONG (4) 1<4096>
Orientation (274) SHORT (3) 1<1>
RowsPerStrip (278) LONG (4) 1<2048>
StripByteCounts (279) LONG (4) 1<8388608>
XResolution (282) RATIONAL (5) 1<126.582>
YResolution (283) RATIONAL (5) 1<126.582>
ResolutionUnit (296) SHORT (3) 1<3>
34710 (0x8796) LONG (4) 1<0>
(Tag 34710 is camera information; to make sure this doesn't somehow make any difference, I've zeroed the whole range from the end of the image file directory to the start of data at 0x1000, and that in fact doesn't make any difference.)
I've found the problem - it was in my C program...
I had allocated memory for an array of longs and used fread() to read in the data:
#define PPR 2048;
#define BPP 2;
long *pix;
pix=malloc(PPR*PPR*sizeof(long));
fread(pix,BPP,PPR*PPR,in);
But since the data come in 2-byte chunks (BPP=2) but sizeof(long)=4, fread() packs the data densely inside the allocated memory rather than packing them into long-sized parcels. Thus I end up with two rows packed together into one and the second half of the picture empty.
I've changed it to loop over the number of pixels and read two bytes each time and store them in the allocated memory instead:
for (m=0;m<PPR*PPR;m++) {
b1=fgetc(in);
b2=fgetc(in);
*(pix+m)=256*b1+b2;
}
You understand that if StripOffsets is an array, it is an offset to an array of offsets, right? You might not be doing that dereference properly.
What's your platform? What are you trying to do? If you're willing to work in .NET on Windows, my company sells an image processing toolkit that includes a TIFF codec that works on pretty much anything you can throw at it and will return 16 bpp images. We also have many tools that operate natively on 16bpp images.